The present invention relates to a safety device for a machine, and more particularly to a safety device for a press brake, a folding bench, a stamping machine, a cutting machine or any other machine in which two machine parts are moved towards one another. However, the present invention is not restricted to these applications and can likewise be used in other machines, in which a concurrently moving protective device operating in a non-contact fashion (commonly designated as electro-sensitive protective equipment) can be used for preventing accidents.
DE 202 17 426 U1, which is assigned to the present assignee and which is incorporated here by reference, discloses a safety device, which has in total eight optical barriers running ahead of a moving machine part during its working movement. Four first optical barriers are arranged at increasing first distances in or near the plane of movement, which is defined by the leading edge as a result of the working movement. Four second optical barriers are arranged parallel to the plane of movement, but with an offset to the operator's side. Furthermore, the second optical barriers are arranged slightly higher than the first optical barriers, with the result that each first optical barrier runs ahead of a second optical barrier during the working movement. A particular aspect of this prior art safety device lies in the fact that selected first (and second) optical barriers can be deactivated permanently by means of a configuring means. It is thus very simple to adapt the known safety device to machines having different moving speeds and different overtravels (“stopping distance”). In the case of very slow machines, the overtravels are generally short, and the known safety device in this case only requires a few optical barriers, which are arranged at a smaller distance from the leading edge of the moving machine part. However, in the case of high-speed machines and also in the case of machines having only simple hydraulic drives, the barriers which are close to the leading edge are not capable of stopping the moving machine part in good time before a possible accident. In these cases, the barriers running further ahead are used, too.
In the end, the suitable configuration of the known safety device is thus dependent on the overtravel of the machine. In order to correctly configure the known safety device on a machine, it is desirable to be able to determine the current overtravel. Furthermore, it is desirable for safety reasons to check a selected configuration with regard to the overtravel. In this regard, above-mentioned DE 202 17 426 U1 mentions a program module which is stored in the control unit of the machine and which allows automated determination of the current overtravel. In practice, a CNC control unit is generally used for this purpose, with the moving machine part being moved in a test mode at its working speed and then an emergency stop being triggered via an emergency off button. The current overtravel can then be determined by the CNC control unit using the positions at the time of the emergency stop and once the machine part has come to a standstill.
The advantage of this procedure lies in the fact that the overtravel can be accurately determined by means of the CNC control unit since a CNC control unit is designed for exact position and movement control. In addition, many machines having moving machine parts nowadays have CNC controllers.
A downside of this procedure, however, is the fact that CNC control units are generally not safe as regards the relevant safety provisions, such as the European Standard EN 954-1. Since the determination of the overtravel for the known safety device is a safety-relevant measurement variable, additional measures have to be taken. Generally, additional position sensors, such as switching cams, are required which are used for safety-relevant position sensing alone.